The only cracking I have seen on these decks has been around where the wheels mount, from lots of use, or right where the engine mounts are from a bent crankshaft.

For whatever reason, I've seen a ton of Hondas and Tecumseh powered mowers with bent crankshafts. It doesn't seem to take much. The slight bend in the crankshaft vibrates the mower enough to crack the mounts.